I have a 76 550K and have bought several 76 550 F wiring harnesses. Those are not 100% compatible.
Also, the 76/77 550's have the right hand switch that kills the lights when you press the starter where as 75 has a Off, Low and High beam switch so that part is for sure different. I'm sure you could make them work but if you're looking for a perfect plug and go harness, I don't believe them to be the same.